This invention relates to a method and an apparatus for the out-of-round circumferential machining (particularly by removing material) of out-of-round workpieces, such as piston rings. The apparatus has a tool holder whose radial feed motions are numerically or electronically controlled as a function of the rotary angle of the work spindle; the desired out-of-round contour is stored in a memory.
Lathes for making out-of-round workpieces with numerically or electronically controlled tool holders are known. For example, in German Offenlegungsschrift (Laid-Open-Application) No. 2,006,760 there is disclosed a lathe for an out-of-round machining, having a cutting tool which is fed as a function of the rotary angle of the workpiece and whose feed motion is the sum of the motions of two interconnected, numerically controlled setting motors.
U.S. Pat. No. 4,250,779 discloses an apparatus for the circumferential machining of out-of-round workpieces, particularly piston rings, where the total feed motion of the tool holder is the sum of the superposed motions of serially connected, electronically controlled setting motors. For the operation of this apparatus a pre-programming of the desired out-of-round contour of the workpiece is necessary. Such a desired out-of-round contour may be either traced by a master cam or may be automatically computed.
It has been found in practice, however, that at high cutting speeds (which are desirable for reasons of efficiency and economy) an out-of-round contour stored in a memory could not be realized on the workpiece with satisfactory accuracy without corrections, particularly in case of piston rings where high accuracy is needed since in their installed state they have to face the engine cylinder wall with a small, light-slit like clearance.